1. Field of the Invention
This invention relates to a microwave dual reflector antenna. More particularly, the invention provides a low cost, self-supported front feed reflector antenna with a low sidelobe signal radiation pattern characteristic configurable for the reflector antenna to satisfy rigorous radiation pattern envelope standards, such as the European Telecommunications Standards Institute (ETSI) Class 4.
2. Description of Related Art
Front feed dual reflector antennas direct a signal incident on the main reflector onto a sub-reflector mounted adjacent to the focal region of the main reflector, which in turn directs the signal into a waveguide transmission line typically via a feed horn or aperture to the first stage of a receiver. When the dual reflector antenna is used to transmit a signal, the signals travel from the last stage of the transmitter system, via the waveguide, to the feed aperture, sub-reflector, and main reflector to free space.
The electrical performance of a reflector antenna is typically characterized by its gain, radiation pattern envelope, cross-polarization and return loss performance—efficient gain, radiation pattern envelope and cross-polarization characteristics are essential for efficient microwave link planning and coordination, whilst a good return loss is necessary for efficient radio operation.
Reflector antennas with a narrow radiation pattern envelope enable higher density mounting of separate reflector antennas upon a common support structure, such as a radio tower, without generating RF interference between the separate point-to-point communications links. Narrow radiation pattern envelope communications links also provide the advantage of enabling radio frequency spectrum allocations to be repeatedly re-used at the same location, increasing the number of links available for a given number of channels.
Industry accepted standard measures of an antenna's radiation pattern envelope (RPE) are provided for example by ETSI. ETSI provides four RPE classifications designated Class 1 through Class 4, of which the Class 4 specification is the most rigorous. The ETSI Class 4 RPE specification requires significant improvement over ETSI Class 3 RPE specification. As shown in FIGS. 1a and 1b, the ETSI Class 4 RPE requires approximately 10-12 dB improvements in sidelobe levels over ETSI Class 3 RPE requirements, resulting in a 35-40% increase in the number of links that can be assigned without additional frequency spectrum usage.
Previously, reflector antennas satisfying the ETSI Class 4 specification have been Gregorian dual reflector offset type reflector antennas, for example as shown in FIG. 1c. The dual offset configuration positions the sub-reflector 15 entirely outside of the signal path from the main reflector 50 to free space, which requires extensive additional structure to align and/or fully enclose the large optical system. Further, because of the non-symmetric nature of the dual offset configuration, an increased level of manufacturing and/or assembly precision is required to avoid introducing cross-polar discrimination interference. These additional structure and/or path alignment tuning requirements significantly increase the overall size and complexity of the resulting antenna assembly, thereby increasing the manufacturing, installation and ongoing maintenance costs.
Deep dish reflectors are reflector dishes wherein the ratio of the reflector focal length (F) to reflector diameter (D) is made less than or equal to 0.25 (as opposed to an F/D of 0.35 typically found in more conventional “flat” dish designs). An example of a dielectric cone feed sub-reflector configured for use with a deep dish reflector is disclosed in commonly owned U.S. Pat. No. 6,919,855, titled “Tuned Perturbation Cone Feed for Reflector Antenna” issued Jul. 19, 2005 to Hills (U.S. Pat. No. 6,919,855), hereby incorporated by reference in its entirety. U.S. Pat. No. 6,919,855 utilizes a dielectric block cone feed with a sub-reflector surface and a leading cone surface having a plurality of downward angled non-periodic perturbations concentric about a longitudinal axis of the dielectric block. The cone feed and sub-reflector diameters are minimized where possible, to prevent blockage of the signal path from the reflector dish to free space. Although a significant improvement over prior designs, such configurations have signal patterns in which the sub-reflector edge and distal edge of the feed boom radiate a portion of the signal broadly across the reflector dish surface, including areas proximate the reflector dish periphery and/or a shadow area of the sub-reflector where secondary reflections with the feed boom and/or sub-reflector may be generated, degrading electrical performance. Further, the plurality of angled features and/or steps in the dielectric block requires complex manufacturing procedures which increase the overall manufacturing cost.
A deep dish type reflector dish extends the length (along the boresight axis) of the resulting reflector antenna so that the distal end of the reflector dish tends to function as a cylindrical shield. Therefore, although common in the non-deep dish reflector antennas, conventional deep dish reflector antenna configurations such as U.S. Pat. No. 6,919,855 typically do not utilize a separate forward projecting cylindrical shield.
Therefore it is an object of the invention to provide a simplified reflector antenna apparatus which overcomes limitations in the prior art, and in so doing present a solution that enables a self supported sub-reflector front feed reflector antenna to meet the most stringent radiation pattern envelope electrical performance over the entire operating band used for a typical microwave communication link.